In 1996 in one of SOHO's cameras the shutter remained jammed open for a few hours and this left only a slight burn on the CCD, caused mainly by XUV light that is anyway strongly absorbed by atmosphere and requires vacuum for transmission. But SOHOs sensors are mono and yours is an OSC, therefore it seems plausible, that the colour filter array on top of the CCD (that was made from gelatine/pigment/dye etc) got damaged from the heat from exposure to solar radiation. Most damage would most likely occur at the centre where there is the most intensity of light falling on the sensor and it would take longer to dissipate heat from that part of the CCD.
Another good reason to use mono cameras for astroimaging
